Defect Engineering of Mo_2–<i>x</i>CT_<i>z</i> MXenes through Precursor Alloying and Effects on Electrochemical Properties

Defect engineering in the form of the intentional creation of defects has been shown to enhance the properties of two-dimensional materials in various applications. Herein, we systematically explore a simple and reproducible method for introducing random vacancies and pores in Mo-based MXenes by combining first-principles calculations and experiments. The process is based on alloying Mo₂Ga₂C with Cr, which is an element that, together with Ga, is selectively etched in hydrofluoric acid, resulting in vacancies and vacancy clusters in the MXene sheets. The limit of Cr incorporation on the metal site was found to be approximately 60 atom % in the precursor powder Mo_2-<i>x</i> Cr _<i>x</i> C. Lower concentrations, up to 25 atom %, were used in the subsequent synthesis of Mo_2-<i>x</i> Cr _<i>x</i> Ga₂C, since an increasing Cr content promoted the formation of another MAX phase (Mo_2-<i>x</i> Cr _<i>x</i> GaC). A Mo_1.87CT <i>_z</i> MXene derived from Mo_1.87Cr_0.13Ga₂C (6.5 atom % Cr) exhibited excellent electrochemical behavior, reaching a volumetric capacitance of 1117 Fcm^-3 at 2 mVs^-1 scan rate, and suggested that defect concentration can be used to tune the rate capability. Overall, we have demonstrated that using Cr as a sacrificial element in the MAX phase is a simple and effective strategy for the defect engineering of MXenes. Moreover, this method can likely be extended to include other sacrificial elements and MAX phases, making MXene defect engineering a viable pathway for property enhancement across various applications, including energy storage and catalysis.